Double wall structures are three-layered systems in which the second or intermediate layer is frequently a liquid. The liquid aids in the cooling process when the interior is at high temperature. Examples are double wall steam pipes, pressure vessels and heat exchanger plates. Structural health monitoring and nondestructive testing from the outside, through three layers to the inside wall is difficult. This paper presents a viable solution by proposing the use of ultrasonics to generate a slow guided wave in the structure enabling inspection of the inner wall for flaws. The results of calculations, simulations and experiments are presented and compared. In particular, a two-dimensional model of the setup is introduced and a procedure for obtaining group velocity dispersion diagrams. The model is validated using theoretical and experimental results. Sample dispersion diagrams are presented and compared with those obtained with matrix methods. Finally, the FEM simulation results depict the displacement profiles across the waveguide. The results of both modeling techniques are in good agreement and they provide interesting insights into the wave mechanics of the three-layered waveguide.